Aluminum body and method of making same



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Aug. 1, 1939. F. PAVELKA ALUMINUM BODY AND METHOD OF MAKING SAME Filed June 19, 1957 Fred;-

UNITED STATES PATENT OFFICE ALUIVIINUM BODY AND METHOD OF MAKING SAME Friedrich Pavelka, Klosterneuburg, near Vienna,

Austria, assignor to A. G., Vienna, Austria Radiowerk E. Schrack Application June 19, 1937, Serial No. 149,265 In Austria July 1, 1936 Claims.

My invention relates to a method of manufacturing aluminum bodies having a large reactive surface area.

There is a wide demand in various field of ap- 5 plication for aluminum bodies having a large reactive surface area, i. e.,'the surface area which is active in a physical or chemical process. As examples of such bodies may be mentioned carriers for the catalytic agent in catalytical processes; gas or liquid filters-particularly ultrafilters-and the aluminum material used in flash lamps which material must be capable of being readily oxidized to produce a high luminous emission. Another important application is for electrodes in electrolytic condensers, and electrodes made by the process of the present invention are described and claimed in my copending U. S. patent application, Serial No. 149,266, filed June 19, 1937.

The object of my invention is to provide an aluminum body having a large reactive surface area.

I have found that the size of the crystallites of the aluminum has a considerable influence upon the reactive surface area, and that by reducing the size of the crystallites a very large reactive surface area can be obtained.

In accordance with the invention I mechanically deform an aluminum body to decrease the crystal size to below 0.001 mm., and preferably below 0.0001 mm., and then etch the body, for example with a concentrated strong acid to obtain a porous structure.

(As far as the reduction in crystal size is concerned, the deformation can be effected in several steps by rolling, drawing or the like, provided the heat treatment .usually employed between successive steps is omitted. However, if excesv sive solidification of the material makes the working of the same impossible, I subject the aluminum between successive steps to a short heat treatment at a low temperature, for example at about 270 C., to merely relieve the internal stresses, without increasing the size of the crystallites.

I prefer to use as the starting material, aluminum containing less than 0.2% and preferably less than 0.02% of impurities, because even with very severe deformation such material will still be sufilciently soft to make intermediate heat treatment generally unnecessary.

In order that the invention may be clearly understood and readily carried into effect, I shall describe same in more detail with reference to the accompanying drawin in which the single figure is a perspective view of an aluminum foil made by the method. of the present invention and shows a portion as viewed under a microscope.

In making the foil illustrated in the drawing, I start with a sheet of aluminum having a purity of about 99.99%, a thickness of about 2 mm., and a crystal size of about 0:1 mm. I then reduce the thickness of the sheet by about 90% by rolling the same in as few steps as possible to obtain a foil having a thickness of about 0.2 mm. and a crystal size below 0.001 mm. Particular care must of course be taken that the purity of the foil is not decreased during the rolling process, for example by particles of the roller surface sticking to it.

As the surface of the aluminum becomes oxidized to a more or less degree during the rolling, and during heat treatment if used, I remove the oxide film so formed in order to secure proper etching. In the case of aluminum of high purity, the oxide film can not be dissolved by chemical treatment, for instance with lye, and therefore I remove it mechanically, for instance with a rotary steel brush. This operation must be carried out in a careful manner to avoid the introduction of impurities.

I then etch thesheet with a suitable etching agent, for example by treating the same for about 16 hours and at a temperature not exceeding 20 0., preferably at a temperature of from 10 to 15 C., with concentrated hydrochloric acid, which if desired may be diluted with about 20% of water. After being washed and dried the material has a weight of about of that before etching, whereas its thickness has not changed to any appreciable extent. The material exhibits a porous structure in which the pores and crystal grains are of substantially the same size and less than 0.001 mm.

By varying the time of the etching, 'the foil may be etched only superficially or can be etched throug to obtain a porous structure of great flneness.

The body with a porous structure throughout is very suitable for use as an explosive, an igniting material or the like, when it is impregnated with a liquid or dissolved oxidizing agent such as liquid oxygen or a solution of potassium chlorate.

The body is furthermore very suitable for use in flash lamps in which case it is introduced together with an oxidizing agent and igniting mabulb.

be limited thereto but desire the appended claims to be construed as broadly as permissible in view of the prior art.

What I claim is- 1. In the manufacture of a porous aluminum foil of large reactive surface area, the step of rolling an aluminum sheet in a few steps and without interstep recrystallizing heat treatment to reduce its thickness at least 90% and to reduce the crystal grain size to less than 0.001 mm.

2. In the manufacture of a porous aluminum foil of large reactive surface area, the steps of rolling an aluminum sheet in a few steps to reduce its thickness to at least 90% and to reduce the crystal size to less than 0.00l mm., and subjecting the aluminum to interstep heat treatment ,at temperatures less than 270".

3. In the manufacture of a porous aluminum foil of large reactive surface, the steps of rolling in several steps and without interstep heattreatment a sheet of aluminum 01' a thickness of 2 mm. to reduce its thickness to about 0.2 mm. and to reduce its crystal size to less than 0.001 mm. Y

4. A porous aluminum foil of large reactive surface and having a purity greater than 99.8% and comprising crystals of a grain size less than .0..001,mm., said foil being porous throughout with a pore size substantially equal to the size of the crystals.

5. A porous body for use as a combustible material in flash lamps comprising aluminum containing less than 0.2% of impurities and having crystals of a grain size less than 0.001 mm., said body being porous throughout with a pore size substantially equal to the crystal size.

FRIEDRICH PAVELKA. 

